One of the primary means used to diagnose the state of a person's health is to analyze the person's blood or other bodily fluids for the presence or absence of "analytes" indicative of the person's health. An "analyte" is any substance contained in a bodily fluid for which either knowledge of the presence or concentration is desired for clinical or laboratory purposes, or for which the substance itself is retained and used for some other analytical or pharmacological purpose. Analytes such as hormones, salts, enzymes, ketones, glucose, drugs, infectious agents, etc., indicate whether the person has contracted an infection and/or whether the person's glands and/or other organs are functioning effectively.
Certain bodily fluids, such as urine, saliva, semen, etc. can be obtained non-invasively, and analysis of such fluids can provide information about the state of a person's health. Some analytes will be found only in certain bodily fluids, such as cerebrospinal fluid. However, the bodily fluid that typically gives the most complete picture of a person's health is whole blood.
One difficulty encountered in analyzing many bodily fluids, such as whole blood, is that the "fluid" actually is composed of both solid and liquid components. For example, whole blood is composed of two fractions. One fraction is a "cellular" fraction, which includes the various types of blood cells and any other relatively "solid" matter found in the blood. The other fraction is an "acellular" fraction which consists of plasma or serum. Most of the "analytes" of interest are contained in the acellular fraction.
In order to obtain the most accurate analysis of the blood, it is important to separate the analytes from the cellular fraction of the blood relatively quickly after the sample is drawn. Otherwise, the quantity and or quality of various analytes in the blood may be altered by the cellular fraction, e.g., by metabolism, release of intracellular content, and/or simple dilution.
For similar reasons, the manner in which the cellular fraction is separated from the acellular fraction of the blood is important in order to avoid contamination of the acellular fraction with components from the cellular fraction. For example, the separation process, itself, can cause the cells in the sample to lyse and release the intercellular content into the sample, thereby contaminating the sample.
Methods for separating clinically useful volumes of blood generally require the blood to be collected, transferred into a glass or plastic vial, and then centrifuged at high speed for a period of time. Centrifugation causes the cellular or solid fraction of the sample to collect at the bottom of the vial and the acellular supernatant plasma or serum to collect at the top of the vial. The supernatant then can be decanted and analyzed. Sometimes, the vial is provided with a medium density inert gel which--due to its specific gravity--substantially isolates the cellular from the acellular fraction when centrifuged, permitting the components to be frozen and stored together.
Centrifugation and subsequent refrigeration of samples is sufficient to preserve the integrity of a sample of blood or other bodily fluid for a sufficient period of time in most cases. However, the process is cumbersome and expensive because it requires the blood or other sample either to be collected in a location having all of the proper equipment, or that the sample be refrigerated and transferred to a separate laboratory for further processing. Also, a substantial amount of time is required to obtain the results of the analysis. Time may be critical to the life or health of the patient; therefore, a more rapid, less expensive, less cumbersome method is needed to analyze samples of blood and/or other bodily fluids.
Some methods exist for separating and analyzing very small volumes of blood or other bodily fluids in situ. Typically, these methods involve exposing the blood or other sample to a porous membrane or a porous mat of glass, fiber, or a polymer of plastic, protein, or carbohydrate, which traps the cellular components (or solids) but permits the plasma to flow through onto a paper or plastic strip where the plasma is reacted immediately with analytical reagents.
Unfortunately, current methods and devices used for in situ analyses produce only a very small volume of serum or plasma which can be used only to measure one or a limited number of small analytes, such as glucose, electrolytes, and pH. The sample of blood must be small--typically less than 100 microliters. Because the entire sample is reacted immediately with the testing reagent(s), the entire sample is disposed of after it has been used for this limited purpose. None of the sample can be stored or used for any other testing.
The in situ separation devices in current use often either do not separate the sample into cellular and acellular fractions, or they separate the serum into subfractions, thereby altering the biochemistry of the serum. Also, the type of analyses that can be performed using such devices typically is a one stage procedure. The analyte is mixed with one or a number of reagents simultaneously to produce a color change or chemical reaction. Because these devices lack a supportive matrix and a path through which the sample can pass before further analysis, they cannot be used to perform a multiple stage analysis capable of measuring complex molecules, such as hormones, enzymes, antibodies, or viral particles. In addition, the accuracy of such analyses is impaired by the presence of hemoglobin, which is freed by the hemolysis or rupture of red blood cells in the sample.
The current methods used to test bodily fluids in situ also lack a convenient means for prolonged storage of viable samples. Currently, samples of bodily fluids must be refrigerated at or below -20.degree. C. in order to maintain the biochemical integrity of the sample. If the sample is to be tested for certain short lived or delicate analytes, preservatives and/or acids often must be added to preserve the integrity of the analytes.
Some tests permit whole blood samples to be stored on blotting paper dried in air--for example, for neonatal purposes. However, these dried samples: can be used only to test qualitatively for a limited number of analytes; cannot be used for quantitative testing; and, only remain viable for testing for about two weeks. The short term viability for such dried samples largely results because the blood cells tend to rupture and contaminate the sample with intracellular contents during the drying process. The free hemoglobin released in the rehydration process also interferes with many colorimetric assays.
Some analytes, such as insulin, can be preserved in a desiccated state by freeze drying the analyte under a vacuum. Unfortunately, a significant number of analytes that can be preserved in a desiccated state denature at some point during the process. Therefore, freeze drying of analytes is only of limited use to test for analytes that do not denature.
An accurate, efficient process for in situ collection, separation, testing, and storage of bodily fluids, such as blood, which can be used for a broad spectrum of analytes would be highly desirable.